Recently so called "wheel lift" towing systems for coupling a disabled vehicle to a tow truck have gained increased popularity. This is particularly true with the advent of front wheel drive vehicles and the use of air spoilers, plastic panels and like structural developments in automobile design which are susceptible to damage when using more conventional chains, cables, straps, slings or similar "wrecker" apparatus which hook onto the vehicle's bumper, undercarriage frame, axles and the like.
While known "wheel lift" towing mechanisms are designed to engage the vehicle's wheels and elevate one end of the vehicle for transporting by a wrecker tow truck, such systems are not without their problems and shortcomings. Until recently, for example, known "wheel lift" systems were unable to compensate for uneven surfaces and ground levels or grades at the sight of the vehicle to be towed. If the ground slope were at a severe enough angle relative to the wrecker, it made it very difficult, if not impossible, to couple the wheel restraints to the disabled vehicle's wheels. In answer to this problem, some systems have incorporated a powered tilt cylinder assembly for tilting the outer lift boom of the "wheel lift" apparatus. This enables the wheel engaging restraints to be placed at various angles vertically in order to line up with and engage the disabled vehicle wheels. While this modification may be generally satisfactory from a functional standpoint, the additional weight and cost of the tilt cylinder assembly is not acceptable, particularly at the rear extremity of the tow truck where its weight has maximum unbalancing effect.
Of similar consequence has been the prevalence in past "wheel lift" mechanisms to employ pivotal lift masts and powered lift cylinder arrangements or winch hoists for elevating the towed vehicle once its wheels have been coupled to the towing mechanism. Examples of such prior devleopments are found in U.S. Pat. Nos. 3,599,811, issued Aug. 17, 1971; 3,667,630, issued June 7, 1960; 4,000,823, issued Jan. 4, 1977; 4,451,193, issued May 29, 1984; and 4,473,334, issued Sept. 25, 1984, among others. Aside from size, weight and cost factors, one of the major detractions of such mechanisms is the need for sufficient head room to permit elevation of the mast while maintaining sufficient clearance of overhead obstructions, such as bridges, underpasses and like obstacles to provide for passage of the towed vehicle.